Recently, network convergence has been promoted. With regard to digital wrapper layers of the OTN (Optical Transport Network) apparatuses and/or the ODU (Optical channel Data Unit) apparatuses provided in the recommendation G. 709 of ITU-T (International Telecommunication Union-Telecommunication sector), a demand as follows has arisen. That is, the demand for a system (such as an OTN Switch (cross connect) and an ODU Switch (cross connect)) having a switching function such as a TSI (Time Slot Interchange) function of SONET (Synchronous Optical NETwork) has arisen.
As for the ODU switching system, many methods of implementing the above mentioned function using an existing packet switch or segment switch are proposed. That is, an ODU frame which has been received from an OTN is fragmented (packetized or segmented using an NPU (Network Processor Unit)). The reason why the frame is fragmented by packetization or segmentation lies in the fact that effective utilization of the existing packet switch and segment switch is realized by fragmenting ODU frames of different formats into general packets or segments. “Packetization” means to fragment a frame into pieces in accordance with an existing standardized packet format and “segmentation” means to fragment a frame into pieces in accordance with a particular format.
In addition, in the OTN for which high reliability of a bearer network is required, a redundancy configuration using a normally used system (Work) and a spare system (Protect, PTCT) (hereinafter, referred to as a Work system and a Protect system) is frequently adopted.
FIG. 1 is a diagram illustrating an example of a data processing apparatus which is connected to an OTN which is configured to have redundancy. In the example illustrated in FIG. 1, the OTN installed between a data processing apparatus 1A and a data processing apparatus 1B has a redundancy configuration constituted by an OTN-W of the Work system and an OTN-P of the Protect system.
Packet data is input from a client into the data processing apparatus 1A using a client interface unit 2A. The client interface unit 2A is then connected with network interface units 4A-W and 4A-P via a switch unit 3A to distribute the packet data to the network interface units 4A-W and 4A-P. The network interface units 4A-W and 4A-P then generate OTU (Optical channel Transport Unit) frames from the input packet data and transmit the generated OTU frames to the OTN-W of the Work system and the OTN-P of the Protect system.
The OTU frames are then input into the data processing apparatus 1B from the OTN-W of the Work system and the OTN-P of the Protect system using network interface units 2B-W and 2B-P respectively. And the network interface units 2B-W and 2B-P then fragment the ODU frames from which OTU overheads have been removed into pieces using NPUs. A switch unit 3B connects the network interface units 2B-W and 2B-P with a client interface unit 4B. The client interface unit 4B performs buffering on fragmented pieces of data sent from the network interface unit 2B-W and fragmented pieces of data sent from the network interface unit 2B-P respectively. The client interface unit 4B selects either of the fragmented pieces of data, restores the selected fragmented pieces of data to the previous ODU frame, adds an OTU overhead to the frame to form an OTU frame and then transmits the formed OTU frame to another OTN apparatus. In normal operation (state), the client interface unit 4B selects the fragmented pieces of data sent from the network interface unit 2B-W. When a fault occurs in the Work system or when the Work system is in maintenance, the client interface unit 4B selects the fragmented pieces of data sent from the network interface unit 2B-P as a protection function. Incidentally, in the example illustrated in the drawing, another OTN apparatus is not configured to have redundancy as it does not constitute a bearer network. However, it may be possible to configure it to have redundancy by branching the data into pieces at the succeeding stages of the network interface units 2B-W and 2B-P and adding a switch unit and a client interface unit thereto.
On the other hand, with the progress of a semiconductor micro-patterning technique, the signal capacity processed using a single interface card is increased. Therefore, the range influenced by a line error which would occur upon switching from the Work system to the Protect system to be performed using the interface card in maintenance is being increased accordingly.
A demand for short-interruption-free (errorless) switching to be performed to avoid the line error as described above is being increased and realization of a system having an errorless switching function is being asked.
Japanese Laid-open Patent Publication No. 6-327074 discloses a technique for preventing cell loss and frame discard from being generated by performing system switching without stopping reception of a packet cell when system switching is performed on a system having a redundancy configuration.
Japanese Laid-open Patent Publication No. 5-130141 discloses a technique for compensating for the sequencing property of packets by buffering information on a time at which a packet is sent out from a transmitting end and information on a time at which the immediately preceding packet has been sent out at a receiving end by packet transmission which is configured to have redundancy, rearranging the packets in time order and then outputting the rearranged packets.